In joint user pairing and resource allocation, scheduling is considered for downlink (DL) multiuser (MU) multi-input-multi-output (MIMO) with linear precoding in which a base station schedules several user terminals to transmit at the same resource block (RB), based on certain channel information available at the base station. Several practical constraints are considered including common rank constraint, maximum number of layers allowed for MU pairing on each RB, and maximum number of layers per user allowed for MU-MIMO transmission on each RB, where the rank or the transmission rank of a user denotes the number of data symbols that are simultaneously transmitted before precoding or the number of columns of the linear precoding matrix for a user. The dynamic switching between single-user (SU) MIMO and MU-MIMO should be supported by the scheduler.
The MU-MIMO scheduling problem can be posed as an optimization problem to maximize the weighted sum rate over all available RBs as the objective function. However, with the above practical constraints, finding the optimal solution of such problem becomes non-deterministic polynomial-time hard (NP-hard).
A greedy algorithm (method) and a genetic method for MU-MIMO user pairing are presented in “Genetic and Greedy User Scheduling for Multiuser MIMO Systems with Successive Zero-Forcing,” by Elliott et al., in Proc. IEEE GlobeCom, Honolulu, Hi., 2009, which only considers the narrowband MU-MIMO user pairing. The methods described by Elliot et al. do not solve the problem of MU-MIMO scheduling for wideband MIMO-OFDM with common rank constraint over all scheduled RBs.
With the rank restriction, the transmission rank for each user is then determined. The optimal user pairing and greedy method can be applied to wideband scheduling but it suffers the loss on the throughput performance.
Accordingly, improved methods are needed to achieve better throughput performance.